Underground coal gasification (UCG) is a technique for extraction of energy contained within a coal seam. This method is a viable alternative to conventional mining by reducing the surface footprint, emission, and energy costs. In addition, UCG is useful in harvesting of energy in coal seams unsuitable for conventional mining techniques. UCG, in its simplest form, involves drilling a production well and an injector well from the surface into an existing coal seam. These are linked together horizontally by drilling, fracturing, or combustion links. Once the wellbore has been drilled, casing is lowered into the wellbore. A cementitious slurry is then introduced into the wellbore and is pumped down the inside of the pipe or casing and back up the outside of the pipe or casing through the annular space between the exterior of the casing and the borehole. The cement slurry is then allowed to set and harden to hold the casing in place. This effectively seals the subterranean zones in the formation, called “zonal isolation” and supports the casing.
Once drilling and cementing of the wells are complete, the coal is ignited underground, air or oxygen-enriched air (and sometimes also water) is then introduced through an injector well. The air reforms with the combustion materials (coal) and forms a synthesis gas (syngas) containing carbon monoxide, hydrogen and methane (as a minor component). The combustion front consumes the coal seam. The syngas moves under pressure through the coal seam to the production well, where it travels uphole to the downstream facility. The stream of syngas coming through the producing well to the surface can be used to drive a turbine and generate electricity. Furthermore the syngas is used as a chemical feedstock or as a fuel for power generation.
UCG offers several inherent advantages over conventional mining, including avoidance of the environmental impact which occurs during strip mining of coal, avoidance of problems of spoil banks, slag piles and acid mine drainage, reduction in emissions, lower energy costs and avoidance of safety and health hazards related to the underground mining of coal.
In many areas, UCG wells are very shallow and the majority of the coal seams are located very near the surface. In such instances, the wells may be drilled such that they penetrate the coal seam by only a few meters. As such, only a small portion of the cement sheath is exposed directly to the combustion front.
Since coal typically resides at shallow depths, it is often necessary to introduce into the wellbore a cementitious slurry that sets at relatively low bottomhole static temperatures. Typically, it is desired that the cementitious slurry set at low bottomhole static temperatures. In addition, it is important that the set cement has the ability to withstand the extreme temperatures of the advancing combustion front. Typically, the temperatures of the combustion front are in excess of 800° C. While Portland cement has been used in geothermal wells in which production temperatures can reach greater than 380° C., such cements typically disintegrate around 450° C.
More recently, cements containing calcium aluminate phosphate (CaP) have been proposed for the cementing of the casing strings for UCG applications. Although the CaP cement can typically withstand the temperatures generated by the combustion front and the high production temperatures, it is not ideally suited for everyday cementing operations: Common cementing additives used for Portland cement based systems are unsuitable with CaP cement making it difficult to adjust reliable slurry performances (such as thickening and setting times, fluid loss and free water controls, rheologies) for CaP cement systems. Also, contamination of CaP with Portland cement residues in a cementing unit causes unpredictable setting times. Therefore CaP systems must be handled separately, which requires advanced planning. The expensive logistics and manufacturing, as well as the fact that CaP cements are not available everywhere, significantly increase their costs as compared with Portland cements. The cementing of wells with CaP cements have caused setting failures at the low static temperatures associated with the shallow depth of coal beds, thereby causing failure or incomplete establishment of zonal isolation within the cemented wellbore of the subsurface formations. This causes the undesirable result of gas communication with the surface. Then, costly remedial work is required and lost hours of non-productive time are the consequence.
Alternative cementitious materials have been sought which set at relatively low bottomhole static temperatures and which provide zonal isolation along the majority of the wellbore at average production temperatures. In addition, cementitious materials capable of maintaining integrity of the cement sheath at the combustion front at high temperatures are also desired. Ideally the cementitious materials should be based on Portland cement for which reliable slurry performances for a given wellbore condition can be adjusted with typical chemical additives to achieve a good primary cement job. Besides economics, logistics, and operations are simplified for a Portland cement based system in comparison to a CaP based system.